The fabrication of thick- or thin-film microcircuits entails the deposition of passive components such as resistors or capacitors upon a dielectric substrate. For example, a resistor network is formed by deposition of a resistive ink through a photolithographically prepared stencil. The ink is then dried and fired. The resulting resistance values depend upon the type of ink used, the thickness of the film and the aspect ratio (i.e., length to width ratio) of the rectangular resistors.
For some applications, the design resistance value tolerances are met through process controls during stencil preparation and ink deposition. However, in many applications, narrow resistance tolerances can be met only through the process of gradually reducing the size of the completed resistor until the resistance falls within the design tolerance. This process is often accomplished by initially forming a resistor of a size that yields a resistance of less than a specified value. The size of the resistor is then gradually decreased to progressively increase the resistance to a value within the design tolerance.
Forming the capacitive circuit components involves deposition of a conductive bottom plate, upon which a dielectric film is then deposited, dried and fired. Afterward, a conductive top plate is deposited onto the dielectric film. The bottom and top plates are ultimately connected to the conductor patterns of the circuit. One method for precisely adjusting the capacitance of each component to meet narrow design tolerances is to gradually reduce the top plate area to yield a corresponding reduction in the capacitance. When this method is employed, the capacitors may be originally formed large enough to have values that exceed those specified.
An effective method for precisely reducing the size of either the resistors or capacitor top plates is known as laser trimming. Laser trimming involves precisely directing a suitably powered laser beam against the edge of the resistor or capacitor top plate. The laser beam is maneuvered to gradually trim the edge of the component until the resistance or capacitance falls within the desired tolerance. The resistance or capacitance is continually monitored during trimming via a measurement system that comprises suitably positioned probes and associated test circuitry.
For the sake of clarity, the remaining portion of this discussion will be directed to resistor network microcircuit components fabricated through thick- or thin-film techniques, although it is understood that the discussion is equally applicable to other arrangements and combinations of components in such a microcircuit.
The relative positions of the resistors in a network are fixed according to the configuration of the stencil employed for depositing those resistors. Accordingly, a laser trimming apparatus that includes a programmable controller can be programmed to successively move the laser beam from a trimmed resistor to the edge of the next resistor to be trimmed by using position coordinate data of the resistor locations on the stencil. However, before the component-by-component laser trimming process can be initiated, it is necessary to accurately determine the position of the overall resistor network relative to the coordinate system through which the laser beam moves. That is, the laser beam positioning system, which controls movement of the beam through the coordinate system, must be initialized.
The initialization step is necessary because even though the relative positions of the individual resistors are fixed according to the stencil, the position of the entire network relative to the substrate may vary from one substrate to another because of variations in the deposition process. Hence, the position of the resistor network relative to the laser beam positioning coordinate system will vary. Such variation also occurs when the part carrying the microcircuit does not precisely register in the part-handling assembly of the laser trimming apparatus. Further, the laser beam positioning system must be initialized for each resistor network that is deposited with a single stencil onto one substrate because when more than one stencil is employed with a single substrate, the relative position of the resulting networks on the substrate may vary from one substrate to another.
A prior technique for determining the position of the resistor network relative to the coordinates of the laser positioning system can be referred to as destructive edge-sensing. That technique proceeds as follows. The position of the laser beam is moved to the center of a starting area near a selected resistor. The area is sized and oriented relative to the selected resistor so that subsequent movement of the beam in a predetermined path will always result in contact between the beam and resistor despite variations in the relative position of the network on the substrate, or variations in the registration of the part in the part-handling assembly. For example, a starting area alongside a relatively long resistor with no other components within that area would be suitable. As noted, the position of the resistor relative to the remaining resistors in the network that were deposited with the same stencil is known.
The resistance value of the resistor next to the starting area is monitored by the measurement system. The laser beam is activated and the laser positioning system moves the beam in the predetermined path, generally toward the resistor between the opposing conductor connection points on the resistor. When the beam encounters the edge of the resistor and begins removing resistive material, a change in resistance is detected by the measurement system and the beam movement is immediately stopped. The position of the beam (hence the position of the resistor edge) relative to the coordinate system is noted in the controller.
To complete the process of determining the orientation of the network relative to the coordinate system, it is necessary to detect an edge of a second resistor in the network. Accordingly, the laser beam is moved to another starting area near the edge of the second resistor, that edge being arranged so that it is not parallel to the first-detected edge. The laser beam is then activated and moved generally toward the resistor between the opposing conductor connection points on the resistor. When the beam encounters the edge of the resistor and begins removing resistive material, a change in resistance is detected by the measurement system and the beam movement is immediately stopped. The position of the second edge relative to coordinate system is noted in the controller. With the position of two distinct edges determined, the orientation of the network is readily computed using the resistor position data obtained from the stencil.
After detection of the position of the network relative to the coordinate system, the controller directs the laser beam positioning system to move the beam through a preprogrammed path corresponding to the known position of each resistor in the network. The beam is activated at appropriate intervals for trimming the resistors.
The just-described destructive edge-sensing technique for initializing the laser beam positioning system has at least two shortcomings. First, the system is slow. The beam must move from the starting point toward and into the resistor at a slow rate so that as the edge is encountered and the corresponding resistance change is detected, the beam will still be sufficiently close to the edge of the resistor to provide an accurate indication of the position of that edge. Second, if it happens that the resistors selected for detection by the destructive edge-sensing technique do not need trimming to alter their associated resistances, the destructive edge-sensing technique will damage that part of the circuitry. To avoid this latter problem, it is possible to include a pair of "reference" resistors that may be damaged by the destructive edge-sensing without affecting the overall circuitry. Adding reference resistors increases the cost of the circuit without increasing the speed of the destructive edge sensing technique.